The use of ultrasonic waves for driving away small rodents, such as rats and mice is already known from CH-A 440 821. Here, the sensitive region is still specified to be very wide, namely between 15 and 100 kHz. Over the years, investigations have increasingly been carried out and in particular work has also been carried out by means of sound vibrations in the ultrasonic or in the near-ultrasonic range in order to thereby drive away birds or keep the same away from a space. According to the document DE-A-2708298 one was in this case of the opinion that a plurality of resonators which operate in various resonant frequencies would be necessary in a device to this end, wherein one has overlaid and modulated these signals. In particular, one was convinced that a habituation effect can hereby be prevented. In this device for shying away birds, the at least two sound resonators were arranged above one another in the same radiation direction in a so-called revolving tower, so that a space around the revolving tower should be reachable by means of emission which is as flat as possible and with a sphere of action which is as large as possible. Such devices have been used in particular in agriculture. The sphere of action is on the one hand determined by the sound intensity, but environmental conditions also further influence this sphere of action. These particularly include the geographic structure, planting in the region exposed to sound and also in particular the air movements present.
The high outlay in terms of device technology is only worthwhile however in the case of a bird population which is not characterised by constant alternation of large and small species. The latter necessitates a high outlay in terms of control and sweeping over relatively large frequency ranges, so that “pauses” are created, in which the bird species not affected in each case can settle.
In a later application, the same applicant has stated their earlier considerations and investigations more precisely. Here, they have determined that practically all birds react to frequencies in the range between 20 and 26 kHz. Added to this is the fact that they have determined that the modulation of the sound vibrations acts on the plumage of the bird. Today, one assumes that the perception of the low-frequency vibrations by means of the plumage is a warning system for the birds with respect to a possible attack from the air. The high-frequency sound vibrations lead to vibrations of the skull bone, so that a bird which penetrates a space exposed to sound in such a manner immediately reacts to this in flight and turns around. Practically all devices present on the market today for driving away birds operate in accordance with these principles.
Furthermore, EP-B-0037109 discloses an apparatus for driving away birds and small rodents by exposing at least one space to a first sound vibration, wherein this first sound vibration is modulated with at least one second, low frequency sound vibration, wherein the apparatus comprises a resonator for emitting the sound vibrations, which is rotatably mounted and driven about two shafts which are arranged at an angle with respect to one another. The low frequency component in particular should be perceived via the plumage as disturbing.
Preferably, the space to be exposed to sound should be passed over by the rotating apparatus in as gapless a manner as possible. At or on buildings, the space which can be passed over thereby is structurally limited however.
Since the 70s, these devices have been used much more in cities for building protection, so that the façades of buildings remain spared from bird droppings. Building façades often have a decidedly complex structure and the covering exposure of such a façade to sound is relatively complex. In order to protect such a façade such as for example of historic buildings, such as churches, train stations, castles, etc., a multiplicity of such devices is required. A purely flat emission, as was also possible with the devices according to DE-A-2708298, could in this case no longer be sufficient. Accordingly, one has built this device in accordance with the here interesting closest prior art of EP-0 037 109. Here, a single resonator on the one hand was rotated about a horizontal shaft, which one holds securely in a fork-shaped holder, whilst the fork-shaped holder itself rotates about a vertical shaft. These devices have now been in use for more than years and the maintenance thereof is relatively unproblematic, but just the drive belt has to be replaced from time to time and the devices have to be cleaned and lubricated. Depending on the installation location, one today assumes a maintenance period of approximately 3 to 5 years.
In addition to the complex façade structures, a further problem is the necessary energy supply to the installation locations of the devices. Particularly at historical buildings, one is consequently motivated to place such devices as little as possible and in as unobtrusive a manner as possible, in order to not influence the external appearance and to have a wiring outlay which is as low as possible. Thanks to the resonator, which is rotated about an x-axis on the one hand and about a y-axis on the other hand, a space in front of a façade can now be comprehensively protected. As a result, even the approach of the birds is prevented. It is however problematic that the rotation itself should not have too high a speed about the horizontal axis nor about the vertical axis, as otherwise the sound pattern is too strongly disturbed and loses its effect. In other words, one has determined that at higher rotational speed, although the spatial sound coverage is improved in that barely any birds penetrate the space exposed to sound any more, at the same time the sphere of action has as a result been reduced in size. Due to the increased rotational speed, although it has consequently been prevented that birds settle on a façade protected in such a manner even only for a short time or even also only come within a close area, on the other hand more remote areas were barely still effectively protected. Accordingly, it was necessary to increase the density of the devices on the façade, this has however led to increased costs, to aesthetic problems on listed façades and the higher rotational speed has additionally substantially shortened the maintenance interval. The faster rotating devices must in part be maintained again after one to two years.